1. Field of the Invention
The present invention relates to an encoder.
2. Description of the Related Art
In order to improve the detecting precision of an absolute encoder like an incremental encoder, Japanese Patent Laid-Open No. (“JP”) 2009-198318 forms such an incremental pattern on each track on a scale that a shape or pitch of the pattern periodically changes at a modulation period in a length measuring direction. Thereby, the incremental pattern has not only information of a relative position but also modulation information of an absolute position using a repetitive modulation period. As a result, the absolute position of the scale can be highly precisely detected using information of a highly precise relative position and the less reliable, absolute position information.
JP 2009-198318 provides an incremental signal detecting photodiode array (INC array) and a modulated signal detecting photodiode array (modulation array) as light receiving elements each configured to receive light from the scale. A length of the INC array in the distance measuring direction is made an integer times as large as the modulation length, and a relative position signal representative of a relative position is generated on the basis of an output from the INC array. A modulation pattern position signal is generated on the basis of an output from the modulation array, and used for an absolute position signal.
However, JP 2009-198318 obtains a signal of a different period from an incremental pattern provided on each track, and a pattern detectable range is restricted by the length of the INC array in the length measuring direction.